7/30/2019 High-Capacity and Low-Cost Carbon-Based _Molecular
Basket_ Sorbent for CO2 Capture From Flue Gas
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456r 2010 American Chemical Society pubs.acs.org/EF
Energy Fuels 2011, 25, 456458 : DOI:10.1021/ef101364c
Published on Web 12/10/2010
High-Capacity and Low-Cost Carbon-Based Molecular Basket Sorbent
forCO2 Capture from Flue Gas
Dongxiang Wang, Cigdem Sentorun-Shalaby, Xiaoliang Ma,* and
Chunshan Song*
EMS Energy Institute and Department of Energy and Mineral
Engineering, Pennsylvania State University,
209 Academic Projects Building, University Park, Pennsylvania
16802, United States
Received October 7, 2010. Revised Manuscript Received December
5, 2010
The continuous rise of the atmospheric CO2 concentrationand its
linkage with climate change demand an urgent tech-nological
solution to reduce CO2 emissions.
1 Carbon captureand sequestration (CCS) have been considered as
one of thekey options for mitigating CO2 emissions.
2 Onthe basis ofthecurrent technology (amine scrubbing), the CCS
cost is veryhigh, inwhich the CO2 capturefrom the sourceswas
estimatedto be two-thirds or even more of the total costs for
CCS.3,4
Consequently, many research approaches have been carriedout for
the development of novel technologies to reduce the
cost for the CO2 capture. Among all of these research
efforts,the CO2 capture by adsorption/sorption on the
immobilizedamine sorbents has been considered as one of the
mostpromising approaches.4-9 In our previous studies for
CO2capture, we have developed the novel sorbents, called as
themolecularbasket sorbents (MBSs), which wereprepared
byimmobilizing CO2-philic polyethylenimine (PEI) on
silicamesoporous molecular sieves.10-12 The second generation ofMBS
(MBS-2) prepared by loading 50 wt % PEI on SBA-15showed a CO2
capacity as high as 140 mg of CO2/g of sorbentataCO2 partial
pressure of 15 kPa,because the MBSincreasesthe total density of the
accessible amine functional groups on/in the sorbent.13-15 In
addition, the MBS has also some othersignificant potential
advantages, including high selectivity for
CO2, no or less corrosion problem, high sorption/desorptionrate
because of high gas-sorbent interface area (80 m
2/g),
positive effect of moisture on the MBS performance, andlower
energy consumption during regeneration. However, thesupport
materials currently used in the preparation of the
MBSs were the mesoporous silica molecular sieves, such asMCM-41,
MCM-48, SBA-15, and KIT-6.These materials arecommercially
unavailable right now, and the preparation costof them is very
high. Consequently, it is impractical to use thecurrent MBSs in
mass CO2 capture from the coal powerplants, which produce about
1500 megatons of CO2/year inthe U.S. According to our preliminary
evaluation, the cost ofthe support materials accounts for more than
90% ofthe total MBS preparation cost, indicating that reducing
thecost of the support materials can significantly reduce the
cost
of the sorbent preparation. On the other hand, the mesopor-ous
silica molecular sieves usually show poor hydrothermalstability at
even 100C,
16which mayresult in the degradation
of the sorbent in the cycles. Therefore, it is necessary
todevelop a new generation of MBS with high CO2 sorptioncapacity,
low material cost, and high hydrothermal stability.
With this in mind, the carbon-based support materials
haveattracted our great attention, because many carbon-basedporous
materials with a well-developed pore structure, widerange of pore
sizes, and large total pore volume are commer-cially available17,18
and easy to be prepared from the widelyavailable and low-cost
feedstocks, such as coal or petroleumpitch. In addition, the porous
structure and surface chemistryof carbon-based materials can be
tailored and modified easily.
Some studies on the preparation of CO2 sorbents by loadingPEI on
the carbon materials have been reported in
theliterature.Arenillaset al.reported the preparation of a
sorbentby loading 60 wt % PEI on a fly-ash-derived activated
carbon,which showed a CO2 capacity of 40mg of CO2/g of sorbent.
19
Maroto-Valer et al. reported the preparation of the sorbentsby
loading 33.5 wt % PEI on the activated anthracites. Themeasured
sorption capacity of the sorbent was 26.3 mg ofCO2/g of sorbent at
75 C.
20 Recently, Maroto-Valer et al.reported the loading 39 wt % PEI
on a fly ash to prepare aCO2 sorbent with the measured capacity of
49.8 mg of CO2/gof sorbent at 70 C.
21 Plaza et al. loaded PEI on a commercialactivated carbon to
prepare a sorbent for CO2 capture butfound no positive effect of
PEI loading on the improvement of
CO2 sorption capacity.22 Up to date, all sorbents prepared
by
*To whomcorrespondence should be addressed. E-mail: [email protected]
(X.M.); [email protected] (C.S.).
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Energy Fuels 2011, 25, 456458 : DOI:10.1021/ef101364c
loading PEI on the carbon materials, reported in the
litera-ture, showed much lower CO2 capacity in comparisonto thoseby
loading PEI on the mesoporous silica molecular sieves.
9,15
The objective of the present study is to develop an inexpen-sive
MBS with high CO2 sorption capacity by loading PEI onthe
carbon-based porous materials, instead of the expensive
mesoporous silica molecular sieves.The ultimate purpose is
tosubstantially reduce the MBS preparation cost and, thus,allow the
CO2 capture to be conducted more cost-effectively.
A series of commercial carbon-based materials with differ-ent
pore properties and structures were selected as the sup-ports for
the preparation of carbon-based MBS (CB-MBS) inthisstudy.The
Brunauer-Emmett-Teller (BET) surface area(SBET) of the carbon
samples changes in a range from 1151 to2320 m2/g, and the total
pore volume was in a range from 0.64to 2.93 mL/g. CB-MBSs were
prepared by loading 50 wt %PEI on the carbon samples using the wet
impregnationmethod that was reported in our previous paper.7 The
pre-pared CB-MBS samples were designated as PEI(X)/Y, whereXis the
weightpercentageof the loaded PEI in the sorbentand
Y indicates the support of the carbon material.The CO2 sorption
performance of the CB-MBS samples
was evaluated in a thermogravimetric analyzer (TGA). TheCO2
sorption tests were carried out using high-purity CO2(99.99%) gas
with a flow rate of 100 mL/min at 75 C. Ultra-high-purity N2
(99.99%) was used as a carrier gas for thecleanup of the sample at
100 C before the CO2 sorption andfor desorption of the sorbent
saturated by CO2. The CO2sorption capacity was calculated on the
basisof the changes inthe sample mass. Figure 1 shows the CO2
sorption capacity ofthe CB-MBS samples in comparison to those of
MBS-1[PEI(50)/MCM-41] and MBS-2 [PEI(50)/SBA-15], whichare the
first and second generation of MBSs developed inour
laboratory.10,13,15 In all of the CB-MBS samples, PEI(50)/C4 gave
the highest CO2 sorption capacity, 135 mg of CO2/gof sorbent, which
is higher than the best carbon-based sorbentreported in the
literature21 at thecompatible testconditions bya factor of 2.7.
This value is also higher than that of MBS-1(110 mg of CO2/g of
sorbent, measured at the same con-ditions) by 19% and is almost the
same as that of MBS-2 (138mg of CO2/g of sorbent, measured at the
same conditions).The measured sorption capacity of PEI(50)/C5 is
slightlylower than that of PEI(50)/C4 but significantly higher
thanother carbon materials. It needs to be mentioned that the
CO2capacitymeasured using a TGA with the pure CO2 gas may
beoverestimated in comparison to those measured using a realflue
gas with about 14 vol % CO2. However, the relative
capacity of the sorbents, measured in this study, should be
thesame as those measured using a real flue gas.
Both C4 and C5 are the commercial carbon blacks, and C4has a
SBET value of 1486 m
2/g and a total pore volume (Vtotal)
of 2.93 mL/g. As is well-known, the carbon black has ahierarchy
of structures consisting of particles covalentlybound into
aggregates, which in turn associate by weakinteractions into
agglomerates.23 Interestingly, although theporous structuresof C4
and C5 are quitedifferent fromthatof
SBA-15, which has an ordered mesoporous structure, thesorption
performances of PEI(50)/C4 and PEI(50)/C5 aresimilar to that of
PEI(50)/SBA-15, indicating that the porousmaterial with a structure
of the aggregative particles can alsobeusedas a goodsupportfor
preparing MBS witha highCO2sorption capacity. It was also found
that the performances ofPEI(50)/C1, PEI(50)/C2, and PEI(50)/C3 were
much poorerthan those of PEI(50)/C4 and PEI(50)/C5. C1, C2, and
C3are the commercial activatedcarbons, which are characterizedby
their slit-shaped pore structures. Although the SBET valuesof these
activated carbons are similar to or even higher thanthose ofC4 and
C5, their total pore volumeis less than1.7 mL/g,which may be one of
the reasons why PEI(50)/C1, PEI(50)/C2, and PEI(50)/C3 showed
poorer capacity. The preliminary
correlation between the porous properties and the CO2 sorp-tion
capacity of the prepared sorbents indicates that neitherthe BET
surface area nor the microporous volume but themesoporous volume
plays a more important role in determin-ing the sorption
performance of the sorbents. The detailedcorrelation between the
performance and the textural andporous structure of their support
materials is underway in ourlaboratory for clarifying how the
porous structure of thesupports affect the performance of
CB-MBS.
Interestingly, it was further found that the
volume-basedcapacity of PEI(50)/C4 is even higher than that of
MBS-2 by57% because of the higher packing density (0.35 g/mL) of
theformer than that of the latter. It will significantly reduce
thevolume of the sorbent bed and, thus, reduce the cost for
equipment investment for the mass CO2 capture.The regenerability
of PEI(50)/C4 was also evaluated by
conducting the CO2 sorption/desorption cycles using a TGA.TheCO2
sorption capacity as a function of the cycle number isshown in
Figure 2. A slight reduction of the CO2 sorptioncapacity was
observed with an increasing sorption/desorptioncycle number, but
such a drop trend becomes insignificantwith an increasing cycle
number. About 92% of the initialCO2 sorption capacity can be
recovered after 10 cycles. As iswell-known, the flue gas also
contains moisture and O2. Theeffects of both moisture and O2 were
also examined. It wasfound that the presence of O2 had almost no
effect on thesorption performance of PEI(50)/C4, while the presence
ofmoisture in the model gas had even a positive effect on
thesorption capacity, as the same as that found in our
previousstudy for MBS-2.12 Further investigations in finding a
majorreason for the sorbent degradation and how to improve
thestability of CB-MBS are necessary.
The preparation costof PEI(50)/C4sorbent, which includesthe
costs for PEI, the support carbon material, the consumedsolvent,
and the operation cost for the sorbent preparation atan industrial
scale, was estimated in comparison to that ofMBS-2. The estimated
preparation cost of PEI(50)/C4 isabout $44/kg, while the estimated
preparation cost of MBS-2
Figure 1. CO2 sorption capacity of the prepared CB-MBS samplesin
comparison to MBS-1 [PEI(50)/MCM-41] and MBS-2 [PEI(50)/SBA-15].
Sorption condition: 100% CO2 at 75 C.
(23) Hjelm, R. P.; Wampler, W.; Gerspacher, M. Proc. SPIE
1997,2867, 144147.
